1. Field of Invention
The present invention relates to monitoring radio frequency (RF) emissions within dynamoelectric machines, such as turbine generators, to detect arcing and to provide early warnings of impending failure. More particularly, the present invention uses preexisting sensor leads from thermal sensors as communications media for monitoring RF emissions flowing from the interior to the exterior of a generator by way of such sensor leads. The thermal sensors whose sensor leads are monitored are thermocouples and resistance temperature detectors.
2. Description of the Related Art
Turbine generators are used by electric power utilities to supply electric power to the power distribution grid. These generators are very expensive items of equipment for electric power utilities to purchase and maintain. These generators are costly to repair due to their complexity, large sizes, heavy and bulky components, and due to difficulty in accessing their internal components. In addition, during times when generators are off-line for repair, the utility must purchase from other sources costly replacement electric power. For these reasons, utilities have recognized the importance of early detection of events which may signal impending generator failure. When such an event is identified, the generator is repaired before serious damage and long-term loss of generator service occurs.
Failures in stator windings are a major cause of generator failure. Stator winding failures and other types of generator failures are often preceded by localized arcing across conductors that have broken or have overheated and melted. In addition, arcing is also produced by failed electrical joints or insulation and reduced clearances between components at different potentials. Arcing is recognized as an early warning indicator that severe generator damage may result if corrective measures are not taken.
Generator arcing is also known to generate RF emissions. It is known to those skilled in the art to use RF monitors to monitor arcing activity and to use RF emission data as a basis for decisions on generator repair. FIG. 1 schematically illustrates a generator 10 having three phases 12, a neutral ground lead 14, and a grounding transformer 16. The neutral ground lead 14 is connected to ground by grounding transformer 16. Existing RF monitoring systems use an RF monitor 20, and a coupling device to connect RF monitor 20 to neutral ground lead 14. For example, a high frequency current transformer 22 is disposed with its core around the neutral ground lead 14. Current transformer 22 is used to couple RF currents flowing within the neutral ground lead 14 to RF monitor 20 which is a sensitive RF meter having built in alarm logic and set points. RF monitor 20 is commercially available and model 835A782C-01 sold by the Westinghouse Electric Company is an example of a suitable RF monitor. When a preselected abnormal change in RF activity occurs, such as a step change in RF emission activity, visual and audible alarm signals are activated and maintenance personnel are notified of the abnormal condition.
RF monitors for turbine generators are discussed in detail in U.S. Pat. No. 5,126,677, issued Jun. 30, 1992, entitled "Apparatus and Method for Preventing Spurious Signals to the RF Monitor Used for Early Warning of Impending Failure in Electric Generators and Other Equipment." This patent has a common assignee with the present invention and is hereby incorporated by reference. This patent taught the relationship between source-to-sensor distance and RF signal strength. This patent taught the use of two or more RF sensors and a method of comparing signals received from the two RF emission sensors.
Another example of an issued patent relating to RF monitors is U.S. Pat. No. 5,126,677, issued Aug. 1, 1989, entitled "System and Method for Detecting Arcing in Dynamoelectric Machines." This patent also has a common assignee with the present invention and is hereby incorporated by reference. This patent uses a capacitive voltage coupler attached to the neutral ground lead 14 to provide a connection between RF monitor 20 and neutral ground lead 14.
Unfortunately, the neutral ground lead 14 detects external RF emissions flowing through the ground system. These external emissions originate from most, if not all, of the equipment in the plant. Thus, RF monitoring systems connected to the neutral ground lead 14 suffer from interference from external sources.
The capability of identifying the location of origin of RF emissions is a valuable tool for generator maintenance because such capability can provide maintenance personnel with information which permits more accurate problem diagnosis and faster generator repair. Unfortunately, however, the neutral ground lead 14 receives RF emissions from all sources throughout the interior of generator 10. Thus, RF monitors relying only upon the generator's neutral ground lead 14 are not adapted to identify the particular location within a generator 10 of an RF emission.
A system which uses a plurality of retrofitted RF sensors on generator 10 and the RF source-to-sensor distance relationship mentioned above partially addresses the location identification needs of RF monitoring systems. However, there remains a need for an apparatus and method which requires minimal retrofitting to accomplish the objectives of locating RF emissions at various locations within generator 10. There also remains a need for a convenient apparatus and method which isolates internal RF emissions from RF emissions external to generator 10.
The present invention provides an apparatus and method for RF monitoring which reduces interference from RF sources that are external to generator 10. In addition, the present invention provides an apparatus and method which is a convenient means to indicate locations within generator 10 of the sources of RF emissions. Accordingly, the present invention provides a valuable contribution to generator maintenance.
Prior to providing a detailed description of the present invention, it is appropriate to describe in further detail the components of turbine generators. FIG. 2 schematically illustrates the components of turbine generators that are relevant to the present invention.
Generator 10 has stator 32 which has a tubular shaped aperture adapted to receive rotor 34. Rotor 34, having a tubular shape, extends longitudinally within generator 10 and is surrounded by stator 34. Three phase power leads 12 conduct electric power from generator 10. Neutral ground lead 14 provides a connection to ground for safety purposes.
FIG. 2 identifies two sections at opposite ends of generator 10. These sections are taken along the lines 3--3 and 3a--3a. FIG. 3 is a section view taken along either of the 3--3 or 3a--3a section lines shown in FIG. 2. The views along the 3--3 section line and the 3a--3a section line are essentially the same due to a symmetrical construction of generator 10. Accordingly, FIG. 3 is used to illustrate both sections. In FIG. 3, twelve thermal sensors 36 are shown to be distributed at various locations about stator 32. While twelve thermal sensors 36 are illustrated in the particular geometry of FIG. 3, this illustration is intended to be representative only. Different numbers and configurations of thermal sensors 36 are found among various models of generators 10. Thermal sensor 36 has sensor lead 38 to conduct temperature signals to temperature monitoring equipment (not shown). While only three sensor leads are shown for convenience, each thermal sensor 36 has a sensor lead 38 for a total of twelve sensor leads 38.
Thermal sensors 36 are not typically attached to or installed into stator 32 due to the 20 KVAC maximum voltage on the stator winding. In a gas cooled stator 32, thermal sensors 36 are located near the ending of stator winding bars where they sample the hot gas temperature of the hydrogen passing through the vents built within each bar. In water cooled stators 32, the exit water temperature is measured in the same manner. For a water cooled stator 32 one sensor per bar is required for a total of between thirty-six to seventy-two sensors. However, only twelve thermal sensors 36 are required in a gas cooled stator 32 because of "y" groups in the windings.
Abnormally high temperatures within generator 10 are an indicator of damage or impending damage to generator 10. Thermal sensors 36 allow continuous monitoring of temperature changes within generator 10. Thermal sensors 36 are thermocouples and/or resistance temperature detectors. Thermal sensors 36 are direct current devices, operating in the range of from 0 to 100 degrees C. Now returning to FIG. 2, sensor leads 38 are illustrated leading from both ends of generator 10. While FIG. 2 illustrates a total of six leads (rather than the actual twelve of generator 10), this reduced number of six should be understood to have been chosen to facilitate illustration.
The present inventors have found that useful RF emissions flow on sensor leads 38. It appears that such RF emissions flow on sensor leads 38 for the same reasons that RF emissions flow on the neutral ground lead 14. While sensor leads 38 carry direct current voltages in the range of a few milliamperes, RF emissions are alternating current voltages superimposed upon the direct current voltages.
The present inventors have taken advantage of their finding of RF emissions on sensor leads 38 to provide a novel apparatus and method for monitoring RF emissions. This novel apparatus and method uses preexisting sensor leads 38, either along with or without using the neutral ground lead 14, for monitoring RF emissions within generator 10. Use of sensor leads 38 provides information for locating the source of RF emissions within generator 10. In addition, use of the sensor leads 38 for RF monitoring facilitates reduction of interference from RF emissions originating from outside generator 10. It should be noted that neutral ground lead 14 is directly grounded to the power plant's ground grid and therefore many RF emissions from other systems within the plant are back fed to neutral ground lead 14. Moreover, thermal sensors 36 are preexisting so using sensor leads 38 permits the electric utility to avoid retrofit costs of installing RF sensors within generator 10.